1. Field of the Invention
The present invention relates to a method for preparing an MFI zeolite membrane, a method for controlling the thickness of an MFI zeolite membrane composed of zeolite crystals whose b-axes are all oriented perpendicular to a substrate, and an MFI membrane with variable thickness composed of zeolite crystals whose b-axes are all oriented perpendicular to a substrate.
2. Description of the Related Art
Zeolite is a generic name of a group of crystalline aluminosilicates. Since sites around aluminum bear negative charges in an aluminosilicate skeleton, cations for compensating the negative charges are present within pores of the aluminosilicate skeleton and the remaining space of the pores is usually filled with water molecules. The three-dimensional pore structure, shape and size of zeolite are depending upon the type of the zeolite, and the pore diameter is typically on a molecular scale. Therefore, zeolite is also called as a ‘molecular sieve’ because of its size selectivity or shape selectivity for molecules entering the pores according to the type of the zeolite.
On the other hand, many zeotype molecular sieves are known in which silicon (Si) and aluminum (Al) atoms constituting the skeletal structure of zeolite are partially or wholly replaced by various other elements. Examples of known zeotype molecular sieves include porous silicalite-based molecular sieves free of aluminum, AlPO4-based molecular sieves in which silicon is replaced by phosphorus (P), and other molecular sieves in which the skeletal constituent elements are partially substituted with various metal atoms such as Ti, Mn, Co, Fe and Zn. These zeotype molecular sieves are materials derived from zeolites, and do not belong to zeolite groups based on the mineralogical classification but are commonly called as zeolites in the art. Accordingly, the term ‘zeolite’ as used herein is intended to include the above-mentioned zeotype molecular sieves in a broad sense.
Zeolites with an MFI structure are most actively used and include the following types:
1) ZSM-5: MFI zeolites in which silicon and aluminum are present in a specific ratio;
2) Silicalite-1: zeolites composed of silica only; and
3) TS-1: MFI zeolites in which aluminum atoms are partially replaced by titanium atoms (Ti).
The structure of an MFI zeolite is depicted in FIG. 1. In the MFI zeolite, elliptical pores (0.51 nm×0.55 nm) are connected in a zigzag configuration in the a-axis direction to form channels, substantially circular pores (0.54 nm×0.56 mn) linearly extend in the b-axis direction to form straight channels, and no channels remain open in the c-axis direction.
Powdered MFI zeolites are very widely used in household and industrial applications, including petroleum cracking catalysts, adsorbents, dehydrating agents, ion exchangers, gas purifiers, etc. MFI zeolite membranes supported on porous substrates, such as porous alumina, are widely used as membranes through which molecules can be separated on the basis of size. Furthermore, MFI zeolite membranes can find application in a wide range of fields, for example, second- and third-order nonlinear optical thin films, three-dimensional memory materials, solar energy storage devices, electrode auxiliary materials, carriers of semiconductor quantum dots and quantum wires, molecular circuits, photosensitive devices, luminescent materials, low dielectric constant (k) thin films, anti-rusting coatings, etc.
As described above, the shape and size of pores and the structure of channels in an MFI zeolite vary depending on the direction of the zeolite crystals. Hence, the shape and size of pores and the structure of channels toward a substrate in an MFI zeolite membrane vary according to the direction of the zeolite crystals lying vertically on the substrate. That is, the characteristics of the MFI zeolite membrane are very sensitive to the planar direction of the crystals lying vertically on the substrate. For these reasons, methods for uniformly growing an MFI zeolite membrane in a specific direction, i.e. in the a- or b-axis direction, have been developed in the art. However, methods for the preparation of MFI zeolite membranes with variable thicknesses composed of zeolite crystals whose b-axes are all oriented perpendicular to substrates have not existed to date.
Methods for preparing MFI zeolite membranes on substrates such as glass plates are broadly divided into a primary growth method and a secondary growth method. According to the primary growth method, a glass plate as a substrate is dipped in a gel for the synthesis of an MFI zeolite (hereinafter also referred to as an ‘MFI zeolite synthesis gel’ or simply a ‘synthesis gel’) without any pretreatment to induce spontaneous growth of an MFI zeolite membrane on the substrate. Generally, the gel contains tetrapropylammonium hydroxide (TPAOH). In this case, b-oriented MFI zeolite crystals grow perpendicular to the substrate at the initial stage of the reaction. At this time, a-oriented crystals begin to grow parasitically from central portions of most of the crystals grown on the glass plate. With the passage of time, the crystals grow in various directions, and as a result, the final membrane has various orientations. The randomly oriented MFI zeolite membrane is useful in some applications, but its applicability is limited. Particularly, when the randomly oriented MFI zeolite membrane is applied to the separation of molecules, the molecular permeability, which is one of the most important factors in the molecular separation, is markedly reduced. When an organic base other than TPAOH is used in the primary growth method, no MFI zeolite membrane grows on the substrate. The secondary growth method is an alternative way to overcome the drawbacks of the primary growth method.
According to the secondary growth method, a substrate, to which MFI zeolite crystals are previously attached, is dipped in an MFI zeolite synthesis gel, and then the reaction is allowed to proceed to form an MFI zeolite membrane. The MFI zeolite crystals attached to the substrate act as seeds. The orientation of the MFI zeolite crystals plays an important role in determining the orientation of the MFI zeolite membrane in the subsequent step. For examples the a-axes of the MFI zeolite crystals tend to be oriented perpendicular to the substrate when the a-axes of the MFI zeolite seeds are oriented perpendicular to the substrate, while the b-axes of the MFI zeolite crystals tend to be oriented perpendicular to the substrate when the b-axes of the MFI zeolite seeds are oriented perpendicular to the substrate. The present inventors have conducted leading studies and succeeded in attaching MFI type crystals to various kinds of substrates, such as glass plates, and filed patent applications in Korea and other countries (Korean Patent Application Nos. 2000-0019667, 2000-0064534, 2003-0054157 and 10-2005-0054643, PCT/KR2001/01854, PCT/KR2004/001467, and PCT/KR2005/001960), some of which were patented.
The orientation of the final zeolite membrane is highly sensitive to an organic base of the synthesis gel rather than to the orientation of the seeds. For example, the synthesis gel typically contains TPAOH as the organic base. In this case, although the MFI zeolite seeds are oriented such that the a- or b-axes are perpendicular to the substrate, the orientation of the MFI zeolite membrane varies randomly. Further, even when all seeds are a-oriented and trimer-TPAOH (1-bis-N,N-(tripropylammoniumhexamethylene)di-N,N-tripropylammonium trihydroxide) is used as the organic base, all zeolite crystals do not uniformly grow in the a-axis direction. That is, the orientation of the seeds and the kind of the organic base contained in the synthesis gel greatly affect the orientation of the final MFI zeolite membrane.
The use of the synthesis gel containing TPAOH or trimer-TPAOH makes it difficult to freely control the thickness of the final membrane. For example, 100 nm-thick seeds are attached to the substrate, followed by secondary growth of zeolite crystals using trimer TPAOH for 24 hr to form a membrane, whose thickness is ten times larger than the initial thickness. This procedure is repeated except the use of TPAOH instead of trimer TPAOH to form a membrane whose thickness is twenty times larger than the initial thickness. That is, it is not easy to form a membrane with a desired thickness (e.g., 100 or 150 nm). In the meantime, when a zeolite membrane is used for the separation of molecules, the molecular permeability of the membrane remarkably decreases with increasing thickness of the membrane, which is economically disadvantageous. Thus, there is a need for a technique for the formation of a zeolite membrane as thin as 200 nm that is composed of uniformly oriented zeolite crystals.
Papers and patent publications are referenced and cited throughout the specification, the disclosure of which is incorporated herein by reference in its entirety in order to more clearly disclose the invention and the state of the art to which the invention pertains.